Ultrasonic medical imaging plays a crucial role in modern medicine, gradually becoming more and more important as new developments enter the market. One of the most common imaging applications is echocardiography, or ultrasonic imaging of the cardiac system, providing essential anatomic and functional information on-line. Ultrasonic imaging systems typically produce noisy images, making the diagnosis of these images a task for highly trained expert physicians. One of the most problematic imaging artifacts is clutter, i.e., undesired information that appears in the imaging plane, obstructing the data of interest.
A common method for enhancing the visibility of the desired ultrasonic image relative to the clutter, particularly in patients with low echogenicity (a common phenomenon among obese patients), is administering contrast agents. Such agents enhance the ultrasonic backscatter from blood and aid in its differentiation from the surrounding tissue. This method is described, for example, by Krishna et al., in a paper entitled “Subharmonic Generation from Ultrasonic Contrast Agents,” Physics in Medicine and Biology, vole 44, 1999, pages 681-694, which is incorporated herein by reference.
Using harmonic imaging instead of fundamental imaging, i.e., transmitting ultrasonic signals at one frequency and receiving at twice the transmitted frequency, also reduces clutter effects. Spencer et al. describe this method in a paper entitled “Use of Harmonic Imaging without Echocardiographic Contrast to Improve Two-Dimensional Image Quality,” American Journal of Cardiology, vol. 82, 1998, pages 794-799, which is incorporated herein by reference.
Furthermore, image-processing methods have been developed for detecting clutter-affected pixels in echocardiographic images by means of post-processing. Zwirn and Akselrod present such a method in a paper entitled “Stationary Clutter Rejection in Echocardiography,” presented at the European Study Group on Cardiovascular Oscillations (ESGCO), Leuven, Belgium, May 2004.
An additional class of currently available methods for handling clutter is a family of Clutter Rejection (CR) algorithms, used in Color-Doppler flow imaging. These methods estimate the flow velocity inside the cardiac chambers or other blood vessels and suppress the effect of slow-moving objects; assuming that the blood flow velocity is significantly higher than the motion velocity of the surrounding tissue. These methods are described, for example, by Herment et al. in a paper entitled “Improved Estimation of Low Velocities in Color Doppler Imaging by Adapting the Mean Frequency Estimator to the Clutter Rejection Filter,” IEEE Transactions on Biomedical Engineering, vol. 43, 1996, pages 919-927.
In spite of the tremendous need and advantage for appropriate clutter suppression, there is no clutter suppression method that is devoid of the above limitations.